A Surprise in the Yeast

A brewing program in Philadelphia has come across a wild yeast that not only flocculates clear but also naturally produces lactic acid. Is GY7B the next big thing for sour beer?

John Holl Dec 28, 2017 - 8 min read

A Surprise in the Yeast Primary Image

Upon tasting the beer for the first time, Matthew J. Farber was convinced that he had an infection in his fermentor. The assistant professor of biology and the director of the Brewing Science program at University of the Sciences in Philadelphia had made a pilot batch of beer on a SabCo system in the brewing lab using yeast harvested in the city. Assuming that, and based on previous experiences, the yeast collected—if it was up to the task of fermentation—would be wild, the class regularly uses the Sour American recipe from Michael Tonsmeire’s book, American Sour Beers. It’s a simple lambic-like beer with German Pilsner malt, wheat, oats, and a small amount of Willamette hops.

The students use this recipe because they want to focus on the yeast flavor, says Farber. The batch in question had a clear lactic-acid flavor, something that was not added in the brewing process and that had not shown up in any of the other pilot batches. Further confounding Farber was that the beer was the pale yellow he was expecting but completely clear.

“We spent a lot of time thinking there was a contamination and then spent so much time checking for microbial contamination. When we kept getting the same results, by the fourth batch, we realized that there wasn’t a contamination and that the yeast was making lactic acid in fermentation.”

That’s right. There’s a yeast strain that exists that not only ferments beer but also produces lactic acid. Farber, a long-time homebrewer who has studied brewing for years, realized what a breakthrough this is—and that there will also be skeptics. After he realized that the lactic quality in these batches of beer was coming from the yeast, it was tested and vetted at every level. Now, after months of examination, the university is seeking a patent on the application of the strain and is talking with a some potential partners—they declined to say which ones—in order to get the yeast into the hands of homebrewers and professional brewers alike.


They call it GY7B for now, and it is, indeed, a novel lactic-acid producing yeast capable of acidifying beer to pH 3.5 in as few as five days with final attenuation at two to three weeks.

“It’s ready to go,” Farber says. “I want to share this with the world.”

A Lucky Find

Founded in 2015, the University of Sciences Brewing Science Certificate program is designed to teach the technical and microbiology aspects behind beer. It’s a team-driven teaching model with scientists and brewers teaching students both the practical and real-life scenarios and trends that go into brewing. The program culminates with an internship at a brewery.

During the Microbiology of Beer course, students head out into Philadelphia to find wild-yeast strains capable of fermentation. A research assistant in Farber’s lab found GY7B on a backyard tree in west Philadelphia. It was collected and brought back to the brewery’s lab along with other samples collected by the students. Each strain that is viable for brewing, says Farber, is put through the half-barrel fermentor rather than a coolship to let the students and researchers “dissect what grows.” Characterizing individual yeast for their fermentation character allows a brewer to be more consistent as opposed to the spontaneous fermentation in a coolship. Previously, as you’d imagine, various strains of Brettanomyces have been harvested, imbuing the base beer with the telltale flavors associated with that wild yeast.


When new yeast samples arrive at the University’s brewery, they are placed in a 10-milliliter tube where they are evaluated for CO2 production. They’re then stepped up to a half-liter quick-test fermentation. From there, the program chooses four samples to step up to the next level. As this lacto-producing yeast went through the tests, it “was particularly vigorous and flocculent” and after brewing and testing again was found to be “pleasantly acidic.”

From there, the microbiologists took over, doing media plating to try to dissect what was happening, and all along they kept brewing it over and over, getting the same results. The findings and application in beer were nothing short of a major innovation in brewing. “It gives me goosebumps,” says Farber.

The Taste

“We couldn’t believe it because before this, we didn’t know of anyone brewing with a lactic acid–producing yeast,” Farber said last fall, standing in the brewery while offering a sample of the beer.

The beer has a soft wheat character and a richness from the oats. It’s impossible not to notice the lactic-acid sour snap that is almost savory lemon, but there are also hints of melon and tart green apple that is more natural than an acetaldehyde flaw. The testing of the yeast also revealed glycerol in the mix, which adds sweetness and a full body to the otherwise highly attenuated beer.


The clarity is amazing in the 6 percent ABV ale. The beer flocculates clean, almost crystal clear, giving it the appearance that it has been filtered and centrifuged, something Farber says it has not undergone. 

“This is a novel specimen making unique beer.”

When it comes to all this yeast is capable of doing, “I wouldn’t believe these things if I didn’t have the data to support it,” Farber says.

When the yeast is finally released and gets into brewing systems, it has the opportunity to open up new flavors in beers and, potentially, change the way brewers approach making sour ales.


“It’s a faster, more reproducible yeast, meaning sour beer can be made faster and more consistently, leading to cost savings and making more sour beer available to drinkers,” says Farber. 

A Quick Peek at GY7B

As GY7B works through the intellectual-property process, there are some things university officials can’t discuss, but Farber and the school offered up these specifics:

  • It is a highly attenuating strain (about 95 percent) that yields beers with bright lactic acidity balanced by a slight sweetness and mouthfeel from glycerol produced during fermentation.
  • Perfectly complementing the tartness of the beer is a pleasant bouquet of apple esters with no discernable off-flavors.
  • GY7B performs well by itself or in co-fermentation with other ale yeast.
  • The high-flocculation character yields a brilliantly clear beer without filtration.
  • GY7B presents the opportunity to quickly brew a delightful sour beer without the limitations and risk of using bacteria.

GY7B fermentation stats:

Flocculation: High


Alcohol tolerance: Medium (4–8 percent)

Type: Sour beer 

Attenuation: 95 percent 

Optimum fermentation temperature: 68–72°F (20–22°C)

John Holl is the author of Drink Beer, Think Beer: Getting to the Bottom of Every Pint, and has worked for both Craft Beer & Brewing Magazine® and All About Beer Magazine.